With the introduction of the continuous cold reduction strip metal mill in which the trailing end portions of a strip moving through the mill is welded to the leading end portion of a strip being led off a new coil, which operation requires a strip storage system for the trailing end portion of the strip in the mill in order to keep the mill running while the weld is made, the previous close control of the lateral position of the strip entering the cold mill from a back tensioning strip pay-off reel was lost. This is especially the case because all hot rolled strip being fed to cold reduction mills has some camber, most notably at the leading end portion of the strip, and this camber results in the strip leaving the strip storage system being off center relative to the center line of the mill.
Prior to applicant's invention, this difficulty was remedied by passing the strip from the strip storage system into a looping pit where no tension is present in the strip and as the strip comes out of the looping pit, edge guides can steer the moving strip into centered position relative to the center line of the mill. However, this arrangement has the fault that the strip entering the first mill stand back tensioning unit has no appreciable tension on it and since tensioning units used in cold reduction mills merely multiply the tension on the strip entering them, the strip going from the back tensioning unit to the first mill stand in such case cannot be at the tension necessary for a satisfactory percentage of cold reduction in the first mill stand. As is well known, in cold reducing metal strip, the cold reduction operation in each mill stand results from a combination of the crushing force of the work rolls and the elongating force of the tension on the strip. Therefore the remedy for the off center strip difficulty in a cold reduction mill prior to the applicant's invention, necessarily resulted in a first mill stand which could not perform the desired percentage of cold reduction expected from each mill stand and instead of having, for example, a five stand mill, the over all effect was to have a four stand mill.
Additionally, edge guides necessary with the prior arrangement often damage the edges of the strip and are themselves, being formed of brass or the like, cut and grooved by the strip. The former fault means damaged product unsaleable without edge shearing and the latter means appreciable maintenance costs.
Applicant has eliminated these problems and has provided a continuous cold reduction metal strip mill in which all the mill stands perform desired percentages of cold reduction. This has been accomplished by maintaining the strip issuing from the strip storage system at the desired high speed strip handling tension, omitting the looping pit and taking the taut strip metal directly from the strip storage system into the back tensioning unit ahead of the first mill stand. Since the strip metal may be off center relative to the center line of the mill as the strip comes from the strip storage system, applicant has provided means for pivotal movement of the back tensioning unit around a vertical axis (such device being herein sometimes referred to as a pivoting, running strip gripping means) to thereby align the entering strip with the center line of the mill. A laser beam strip tracking device between the back tensioning unit and the first mill stand can be utilized to supply signals to power means for actuating the pivotal movement of the back tensioning unit to maintain the strip entering the first mill stand centered.
Although this invention is applicable to all metals which are cold reduced under elongation tension, it will be described in the environment of the steel industry.